1. Field of the Invention
The present invention relates to an image transferring device for an image forming apparatus such as a copier, printer, facsimile transceiver or similar photographic image forming apparatus in which an image is formed on a photoconductive element. More particularly, the invention is concerned with a contact type image transferring device including, for example, a transfer roller or a transfer belt for transferring a toner image from the photoconductive element to a sheet of paper which is passed through a nip between the photoconductive element and the image transferring device. The present invention further relates to a method and apparatus for electrically cleaning the transferring device.
2. Discussion of the Background
It is a common practice for an image forming apparatus of the type described above to use a contact type image transferring device. The contact type image transferring device such as a transfer roller has applied thereto an electrical field opposite in polarity to the polarity of a toner image on a photoconductive element. The image transferring device transfers the toner image from the photoconductive element to a sheet passed through a nip between the photoconductive element and the transfer device. Since the contact type transfer device is in direct contact with the photoconductive element when the sheet is not at the nip, the toner image on the surface of the photoconductive element transfers to the surface of the transfer device. Subsequently, the toner image on the transfer device is undesirably transferred to the back side of the sheet.
Japanese Laid-Open Patent No. 3-69978 discloses a cleaning device for a transfer roller in which toner on the surface of the roller is transferred to the photoconductive element by applying cleaning bias voltage to the transfer roller when the transfer roller is in direct contact with the photoconductive element. Namely, the cleaning bias voltage is applied during a pre-image forming time period (i.e., from the time the photoconductive element starts its rotation until the leading edge of an image area on the photoconductive element reaches the nip), an inter-image forming time period (i.e., between successive copying or toner image transfer operations), and a post-image forming time period (i.e., after the last image area on the photoconductive element passes through the nip). Since there is not only regularly charged toner having a positive polarity but also oppositely charged toner having a negative polarity, for cleaning both types of toner, this publication discloses that the polarity of a cleaning bias voltage is switched over between the positive polarity and the negative polarity.
Japanese Laid-Open Patent No. 51-9840 discloses a cleaning device for a transfer roller in which the transfer bias voltage having a polarity opposite to the polarity of the toner is applied to the transfer roller when a sheet of paper is at the nip, and the cleaning bias voltage having the same polarity as the toner is applied to the transfer roller when the sheet of paper is not at the nip.
Japanese Laid-Open Patent No. 2-39182 discloses a cleaning device for a transfer roller in which a cleaning bias voltage having the same polarity as the transfer bias voltage and a voltage value smaller than the transfer bias voltage is applied to the transfer roller when a sheet of paper is not at a nip between the transfer roller and a photoconductive element.
It is known that there is oppositely charged toner in a developing device. The oppositely charged toner is adhered to a non-image forming area of a photoconductive element when the photoconductive element faces a developing roller in the developing device. The oppositely charged toner on the photoconductive element is undesirably transferred to a transfer roller when an area of the photoconductive element where the oppositely charged toner is adhered reaches a nip between the photoconductive element and the transfer roller after the cleaning operation for the transfer roller is completed. When a sheet of paper passes through the nip, the oppositely charged toner on the transfer roller is undesirably transferred to a back side of the sheet of paper, and then the back side of the sheet of paper becomes dirty. Even if the cleaning operation for the transfer roller is executed during the cleaning time period, the aforementioned drawback cannot be solved, since this drawback occurs after the cleaning operation of the transfer roller. Therefore, the aforementioned publication does not solve this problem.